Electrokinetic (EK) remediation technology is a method of remediating heavy-metal contaminated soil by applying a low-density direct current across the soil. The EK process mainly involves two processes: electromigration (charge flow) and electro-osmosis (pore fluid flow). Electromigration is the process where ions are transported to the opposite charged electrode. Electro-osmosis is the convection flow produced by the transport of excessive cations in the electric double layer (EDL) of colloid particles. Both processes are unsustainable in long-term EK treatments. Even though methods such as polarity reversal, voltage gradient enlargement and water replenishment had been proposed and applied, the decrease in electric current and the unsustainable electro-osmosis fluid were still observed during the long-term EK treatments.

Dr. CHENG Fenglian, Prof. GUO Shuhai and their colleagues at the Institute of Applied Ecology, Chinese Academy of Sciences, recently studied the changes in soil ions and colloid properties during the EK remediation process.

The researchers found that the decrease in electric current with time could be divided into three phases. The researchers elucidated that the relationship between electromigration and electroosmosis processes in these time-phases and revealed the reason why soil ions were spatially heterogeneous under electric fields.

Electromigration and electro-osmosis induced the changes in temporal and spatial distribution patterns of soil pH and water-soluble ions were found to be responsible for the changes in thickness and particle size of the colloid electric double layer in different sections of the soil. Furthermore, the researchers pointed out that the deficiency of mobile cations and the aggregation of soil colloid particles were main reasons affecting the effectiveness of the EK process. “Appropriate electrolytes to condition the soil colloid properties in different phases and soil sections can be used to maintain the EK remediation,” the researchers said.

The study, titled “The loss of mobile ions and the aggregation of soil colloid: Results of the electrokinetic effect and the cause of process termination” has been published in Electrochimica Acta. The study is finically supported by the National Science and Technology Major Project of the Ministry of Science and Technology of China and the National Natural Science Foundation of China.